Ultra-low and cryogenic temperatures ranging from -95.degree. C. to -150.degree. C. have been achieved in refrigeration systems using a single circuit vapor compressor. These systems typically use a single compressor to pump a mixture of four or five CFC containing refrigerants to reach an evaporative temperature of as low as -160.degree. C.
Environmental concern over the depletion of the ozonesphere has increased pressure on refrigerator manufacturers to substantially reduce the level of CFC-containing refrigerants used within their systems. Although non-CFC refrigerant mixtures have been developed, it has been discovered that most of these refrigerant mixtures cannot simply be substituted for CFC-containing refrigerants in currently available refrigeration systems due to the different thermodynamic properties of the refrigerants.
The present inventor has discovered that using non-CFC refrigerants in conventional ultra-low and cryogenic temperature systems cause an imbalanced flow of the refrigerants in the refrigeration circuit, which reduces the cooling capability of the refrigerants to the compressor. Such low levels of compressor cooling can cause a system to fail due to compressor overheating.
Therefore, the present inventor has developed a novel autocascade ultra-low and cryogenic temperature refrigeration system which is capable of operating with non-CFC refrigerant mixtures. These non-CFC refrigerant mixtures are non-toxic, chemically stable, commercially available and compatible with most of the standard refrigeration oils and compressor materials. Normally, only one component of the non-CFC refrigerant mixture, i.e., hydrochloroflurocarbon (HCFC), is a regulated ozone depleting chemical. As such, the refrigerant mixture typically exhibits an overall ozone depletion factor (ODP) of only 0.06.
Unlike the CFC-containing refrigeration systems which do not cause overheating of the compressor, the present inventor has discovered that the substantially non-CFC refrigeration systems must provide additional liquid return to the compressor in order to avoid overheating thereof and eventual failure of the system.
The present inventor has been able to overcome the overheating of the compressor when using substantially non-CFC refrigerants in a single compressor autocascade system. This is accomplished by providing a specially-designed capillary tube or expansion means disposed downstream of the first liquid/gas separator such that liquid refrigerants are returned directly to the auxiliary condenser and then to the compressor. This feature enables larger than normal quantities of refrigerants of higher boiling points to be rapidly returned to the compressor, which results in excellent operating conditions of the compressor and avoids overheating thereof.
As such, the overall performance of the non-CFC autocascade system is comparable to its counterpart of the CFC autocascade system. This is evidenced by the fact that both systems have similar pull down rates and compressor operating conditions at standard 90.degree. F. ambient.
The present invention also provides many additional advantages which shall become apparent as described below.